Reliable, non-invasive, low-cost, in situ measurement of skull vibration for the purpose of fitting a bone conduction hearing aid currently is not possible. (Hodgetts W. E, Hakansson B. E. V, Hagler P, Soli S, “A comparison of three approaches to verifying aided Baha output,” Internl J Audiology 2009 online.)
Skin placement of an accelerometer as a means to record skull vibration (specifically stimulation created by an implanted bone vibrator) was attempted by Laitakari, et. al. (Laitakari k, Lopponen H, Salmivalli A, Sorri M, “Objective Real Ear Measurements of Bone-conduction hearing Aid Performance,” Scan Audiol 24:53-6, 1995; Laitakari K, Jamsa T., “Computerized in-situ test for bone conduction hearing aids,” Scand Audiol: 30: Suppl 52:79-80, 2001.)
This method uses a typical mechanical headband, like those associated with the oscillators for bone conduction audiometry, to couple the accelerometer to the skull. Currently, it is acknowledged by experts in the field that because vibrations of the overlying skin do not reflect underlying skull vibrations, with the existing art, it is almost impossible to measure skull vibration in living subjects. (Majdalawieh, Osama, Abstract to PhD Thesis, Dalhousie University, Halifax, Canada 2008.)
Miller U.S. Pat. No. 7,447,319—“Method and system for external assessment of hearing aids that include implanted actuators” focused on the output of the hearing aid into an electrical/mechanical detector. A similar approach was taught by Leysieffer in U.S. Patent Application Publication No. 2002/0026091, “Implantable hearing system with means for measuring its coupling quality.” Both of these inventions were not concerned about the in-situ output in a living skull, but rather the output into a device which functions more or less as a “simulator device.”
The dental bone conduction pathway as used herein should be considered a sub-pathway of the non-acoustic “bone conduction pathway” for sound transmission to the hearing nerve. In the dental bone conduction pathway, sound perceived at the hearing nerve originates in structures of the mouth and pharynx. Speech sounds and chewing sounds, for example, travel to the hearing nerve via the dental bone conduction pathway. By contrast, loud ambient helicopter noise that penetrates the skin over the entire skull, neck, and body can be considered noise arriving at the hearing nerve via the bone conduction pathway. Similarly, standard bone conduction audiometry with skull stimulation at the mastoid or forehead uses the bone conduction pathway as distinct from the dental bone conduction pathway.
The distinction between pathways is important because of anatomical differences between the pathways. The bio-mechanical forces in the dental bone conduction pathway are variable and thus may create variable results when compared to stimulation of structures elsewhere on the skull (at the mastoid or forehead for example). The large resonant chamber, anatomically named as the mouth and oropharynx, has its resonance frequency altered by combinations of opening the mouth and movements of the tongue, lips, and vocal chords (human speech). The mere act of biting changes the mechanical load on a top tooth by significantly increasing the load on that top tooth with the effective mass (and variable biting force) from the lower jaw. Other pathway entrances on the skull do not contain such variable effective mass or compliant muscles and ligaments. Also, those other skull areas have far less voluntary muscle and compliant soft tissue (when compared to the tongue and cheeks of the mouth, for example), and more fixed chambers (e.g., frontal sinuses, mastoid air cells, external ear canal), and thus necessarily have more consistent volumes, mechanical loads, and input mechanical point impedances than do structures of the mouth and pharynx; that is, structures comprising the dental bone conduction pathway.
Unlike other methods to record (non-acoustic) skull vibration, the instant invention uses the teeth as the site for measuring skull vibrations. Use of the teeth allows for greater sensitivity because vibration sensing is via the dental bone conduction pathway having no overlying skin, muscle, etc., to affect skull vibration. Equally important, measuring skull vibrations through the teeth allows for repeatable and reproducible tooth placement with a concomitant replication of the (original) coupling force. The use of a tooth microphone for speech recording and voice communication has been taught by Brouns (Brouns J R, “Experimental wide-band tooth-contact microphone,” J Audio Engineer. 9:1:42, 1971); Mersky (U.S. Pat. No. 5,455,842—Underwater Communication System); Anajanappa (U.S. Pat. Nos. 7,269,266; 7,486,798); May (U.S. Pat. No. 5,579,284); Wieland (U.S. Patent Application Publication No. 2009/002235107), and others. The tooth coupling methodologies, associated electronics, and recording software differs substantially from the means and methods taught in this invention. Other art by Saadat and Alboufathi (U.S. Patent Application Publication No. 2009/0281433, “Systems and methods for pulmonary monitoring and treatment”) teaches a means and method for recording breath sounds using an intraoral sensor. That art differs from the instant invention in that it requires an impression of the patient's dentition in order to fabricate and customize the tooth attachment means.